Conventionally known data structures handled by computers include, for example, tabular structures of relational databases, objects of object-oriented databases, and frames or rules of knowledge databases.
However, databases proposed conventionally have a problem in terms of flexibility toward changes, being unable to add a new structure to a defined data structure. For example, when a new attribute is added to one table after a relational database goes into operation, it is necessary to modify an existing data structure and an application program that processes the data structure, and man-hours needed for the modification are burdens on computer users and administrators or application program developers. Also, with conventional relational databases, it is not possible to hierarchically describe information, i.e., for example, define a table further in an attribute of a table. On the other hand, with an object-oriented database or frame, a so-called parent-child relationship can be defined between objects or between frames, but conversely the object-oriented database is sometimes unfit for responding to a demand to process a relationship between an attribute and attribute value in a simple manner.
Thus, in order to allow an information processing device to handle information about things, organizations, persons, and the like handled by a user or concepts and the like handled by the user, the present applicant has proposed a new data structure used to describe information and a procedure for processing information stored in the data structure (see Patent documents 1 to 4). According to this proposal, information is expressed by expression-based representations, i.e., for example, by a sum of an identifier and identifier, a product of an identifier and identifier, a sum of products of an identifier and identifier, and the like.